According to various government websites, a dollar bill costs 5.4 cents to create. A dollar coin costs 18.03 cents to create. Although the coin costs more to create it lasts longer. The dollar bill lasts for approximately 18 months in circulation whereas the coin lasts for 25 years. To keep a dollar bill in circulation for 25 years would cost 86 cents. Once a coin is taken out of circulation the metals can be extracted for additional use making some cost salvageable. Using coins would reduce cost by 68 cents per dollar over 25 years
References:
www.federalreserve.gov/faqs/currency_12771.htm
www.usmint.gov/kids/campcoin/collectorsworkshop/tenfacts.cfm#05
www.secretservice.gov/money_damaged.shtml
www.usmint.gov/faqs/circulating_coins/?action=faq_circulating_coin